Help me build a switch mode powersupply

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Hi All,

Im trying to build a powers supply, with a 15V output and a current of least 30A at 15V.

Anyway, I know how to build linear power supplies as they are pritty simple. Saddly, they are also energy inefficient, heavy and expensive.

I have therefore decided that switch mode is the way to go. The only problem, I dont know anything at all about them! Does anyone have any links which explains how they work, the math involved to calculate the waveforms and the hardware used for the PWM etc.

Thanks

T

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martakz's picture
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I found a nice 15V, 33 amp sw

I found a nice 15V, 33 amp switch mode psu... DROOOOOLLLLLL.

Only £352

Best start saving or build a crappy linear..

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Somebody has to ask: Why?

Somebody has to ask: Why? Smile

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I frequently sell 170-350W pe

I frequently sell 170-350W pelter modules and I have nothing that can provide the 30 amps a 350W peltier needs at 15V...

It would also be nice to have a variable powesupply from 0-15.5V. No more hunting for batteries...

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Well I finally got a decent s

Well I finally got a decent switch mode PSU at a price I could not refuse...£40 Smile Its never been used and supplies 0-15.5V @ 33Amps sustained.

Check it out here: http://www.lambda-gb.com/uk/range_overviews/range_id2.htm

Its the £357 CA400 12F @ 15V. The specs are really amazing for a PSU this powerful.

Im happy Smile Now to hack it, into a nicer case, with variable voltage, current and voltage readings..and to make it a bit safer...

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Lambda makes a darn fine prod

Lambda makes a darn fine product. I've used them for over a decade.

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Thanks for the reply Dr Bob a

Thanks for the reply Dr Bob and could you possible answer a few questions? Smile Im glad its a good company...i certainly dont want it to fail.

1. The powersupply has something called "remote sensing", which apparently measures the voltage at the component being powered and compares it to the output voltage. It then adjusts the voltage to compensate for line loss. Anyway, how does one connect the remote lines? Also, ive heard conecting these wrong is an easy way to blow up a psu...

2. Ive also descovered it has an inrush current of >50Amps. Do you think this will trip breakers / blow fuses in my 240V house?

If so, I'll have to build an inrush limiting circuit, or install some of the the limiting thermistors...

Anyone have a good beginners page on the maths involved to calculate how long it takes for caps to charge etc?

Also, what resistor value should one discharge capacitors in power supplies with? Ive got a feeling this thing will have quite alot of capacitance and hense would prefer to make sure its discharged before playing!

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Re: Thanks for the reply Dr Bob a

martakz wrote:

1. The powersupply has something called "remote sensing", which apparently measures the voltage at the component being powered and compares it to the output voltage. It then adjusts the voltage to compensate for line loss. Anyway, how does one connect the remote lines? Also, ive heard conecting these wrong is an easy way to blow up a psu...

On the back on the lambda supply, there should be two terminals marked S+ and S- that have a jumper between them and another terminal. To use the sense, remove the jumpers, and then connect a small wire (no current will flow through the sense line so a tiny wire is fine) from the S+ terminal to the positive input terminal on your device. Connect another wire from S- to the negative terminal on your device. That's all you have to do. The sense compensation runs all the time, but normally that jumper just makes it sample at the output.

martakz wrote:

2. Ive also descovered it has an inrush current of >50Amps. Do you think this will trip breakers / blow fuses in my 240V house?

Nah, I recently shorted the mains wires on a 208V 30A cable and it took a good 5 seconds to pop the building breaker. It'll make your lights blink though.

martakz wrote:

Also, what resistor value should one discharge capacitors in power supplies with? Ive got a feeling this thing will have quite alot of capacitance and hense would prefer to make sure its discharged before playing!

Don't. The lambda supply physically disconnects its outputs when it's off (or in standby mode).

You should also be aware that those supplies will happily source their rated current into a dead short indefinitely. This means if you mess up, it'll happily melt your prototype into slag with no ill effects on the lambda supply.

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regulation

Say, just how close to +15V do you need to stay? I'm currently designing a power supply that you could build for well under $100 that should get you within 1 or 2 V of 15V with lots of 60hz ripple.

Yea it's dirty, but the device I'm building it for has 120dB ripple rejection at 60hz so I don't care.

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Well I bought the lambda unit

Well I bought the lambda unit for under £100 at £40 Dr Bob. It certainly does not have a 60Hz ripple (well it would be 50Hz in this country) and the ouptuts are near perfectly linear with 2% ripple max.

If intead of building your supply, you could purchase one of these, which are probally better... Be warned, they are not user friendly, use amp connectors and are particually easy to electricute ones self with, as they are designed to power remote equipment. It would therefore be advicable to put them in a different case / not stick your hand in the front.

The weir units are also good @ 100W, they have remote sense too.

Lets just hope the inrush does not blow my fuse box up.

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One last question... from wha

One last question... from what you said, the supply disconnects the outputs when power is off...good. But, if one decided to play around with the powersupply circuit board, then would the capacitors discharge automatically at power off, or could they remain charged, though not connected to the output?

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Re: Well I bought the lambda unit

martakz wrote:

Well I bought the lambda unit for under £100 at £40 Dr Bob. It certainly does not have a 60Hz ripple (well it would be 50Hz in this country) and the ouptuts are near perfectly linear with 2% ripple max.

Nor would I expect the lambda supply to have any significant ripple.

martakz wrote:

If instead of building your supply, you could purchase one of these, which are probally better... Be warned, they are not user friendly, use amp connectors and are particually easy to electricute ones self with, as they are designed to power remote equipment. It would therefore be advicable to put them in a different case / not stick your hand in the front.

For one, the supply I'm building is only 6 amps and only costs about 20 USD to build. For two, my supply is 1/5th the size of the lambda, and For third, I wouldn't learn anything about power supplies if I did that, now would I?

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Re: One last question... from wha

martakz wrote:

One last question... from what you said, the supply disconnects the outputs when power is off...good. But, if one decided to play around with the powersupply circuit board, then would the capacitors discharge automatically at power off, or could they remain charged, though not connected to the output?

Do not under any circumstances open up the supply and screw around inside of it. You cannot make it better than Lambda did.

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Hrm, 6A sounds good, and at ~

Hrm, 6A sounds good, and at ~$20, it sounds better. Your supply design might be useful in a hack to me sometime. I've got that G-Vision LCD I made a post in H.H. about, and it takes 12v @ 5A. It'd be nice to have a seperate supply so I don't have to use a loaded PC AT supply to run it.

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Will your PSU be linear or sw

Will your PSU be linear or switch mode Dr Bob? If its a switch mode, could I possibly have the design?

Also, I don't intend to mess around with the psu board itself, but I need to change the input and output connectors and hense need the cover off.

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Re: Hrm, 6A sounds good, and at ~

Jon wrote:

Hrm, 6A sounds good, and at ~$20, it sounds better. Your supply design might be useful in a hack to me sometime. I've got that G-Vision LCD I made a post in H.H. about, and it takes 12v @ 5A. It'd be nice to have a seperate supply so I don't have to use a loaded PC AT supply to run it.

Just be aware that it's output has a buttload of 60Hz ripple. I won't know exactly how much until I build it and load test it. If your device can tolerate that (as mine can) then this is a great cheap supply.

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Re: Will your PSU be linear or sw

martakz wrote:

Will your PSU be linear or switch mode Dr Bob? If its a switch mode, could I possibly have the design?

Neither, it's a rectifier.

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Why did you choose to build a

Why did you choose to build a rectifier? They are pritty inefficient (if your dropping the voltage how I think you are) and a linear would not cost much more to build. I understand why is has so much ripple now...

Even so, im probally getting a rectifier confused with something else, as your more experienced, hence a rectifier must be most suited for your purposes.

Your dropping your voltage by converting the excess into heat?

Am I right in thinking your doing the following...using a bridge recitifier to get DC, then using a zener with another resistors to drop the voltage across your output to 6V...and using a few capacitors to smooth out the ripple?

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My LCD controller has a power

My LCD controller has a power board that takes the 12v and converts it to 12v & 5v and there are some good sized caps and other components on the board IIRC. I'm assuming there is some filtering going on, but I know little of power supplys.

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Re: Why did you choose to build a

martakz wrote:

Why did you choose to build a rectifier? They are pritty inefficient (if your dropping the voltage how I think you are) and a linear would not cost much more to build. I understand why is has so much ripple now...

Even so, im probally getting a rectifier confused with something else, as your more experienced, hence a rectifier must be most suited for your purposes.

Your dropping your voltage by converting the excess into heat?

Am I right in thinking your doing the following...using a bridge recitifier to get DC, then using a zener with another resistors to drop the voltage across your output to 6V...and using a few capacitors to smooth out the ripple?

I need to take a 120VAC input and generate + and - 26V outputs at 6 amps. I'm using a transformer to reduce the voltage, then a bridge rectifier to make - and + DC rails. I have about 10 millifarads of capacitance on each output rail to smooth it out.

While this approach is not the most efficient it is very cheap and simple. Using a linear to drop from 120 to 28V at 6 amps would generate a GIGANTIC amount of heat and have an efficiency around 25%.

Keep in mind that the circuit I'm powering with this supply has 120dB of input ripple rejection at 60Hz.

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Well I got my lambda today.

Well I got my lambda today. It connects to the mains using crimp plugs. Of cause I only have 2 of these and the supply needs 3!

Anyway, I will have a play.

Btw, Dr Bob, am I wrong in thinking that a linear is just a transformer, bridge rectifier and filter caps? Is the difference between a rectifier and a linear in the way inwhich a linear regulates the output by burning off the excess volts as heat?

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A linear supply uses a transf

A linear supply uses a transformer to lower the voltage (and at the same time raise the current available). A linear regulator uses transistors and resistors to lower the voltage and keep the current constant.

Because of the confusion that often arises between the two, I use the term rectifier as it includes the diode bridge and is generally more descriptive.

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I was at MicroCenter recently

I was at MicroCenter recently. They've got at least 3-4 books on power supplies. A couple looked like references and maybe a couple textbook-ish ones. One of those might be more handy if you try to build one yourself any way.

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